The Ulster Unionists have insisted they remained opposed to the landmark agreement to devolve policing powers to Northern Ireland despite weekend contact from US Secretary of State Hillary Clinton.

Mrs Clinton called UUP leader Sir Reg Empey after his party announced on Friday that it would not back the Hillsborough Agreement at a crucial vote at the Stormont Assembly tomorrow.

Sir Reg said he appreciated the call from Mrs Clinton, but stressed that his party still intended to vote ‘no' on the proposal to transfer law and order responsibilities from London to Belfast in April.

“She (Mrs Clinton) has always taken a very keen interest in Northern Ireland and I thanked her for the call,” he said.

“She's obviously very anxious to see a successful resolution but I explained the situation we faced. She was very pleasant and helpful and I think she understands our view that we should have been more involved (in the Hillsborough talks).”

Sir Reg said “nothing substantive” had developed over the weekend to address any of his party's concerns over the wide-ranging agreement on justice devolution and parades that was hammered out after 10 days of round-the-clock talks between Sinn Fein and the DUP at Hillsborough Castle, Co Down last month.

However, the East Belfast MLA did not close the door on a potential change of position ahead of the UUP executive meeting tonight, when the party will make a final decision on how it will vote.

While Sinn Fein and the DUP have the electoral strength to push through the accord when it is put before the Assembly, a rejection from the UUP will deprive them of the unanimous support they crave.

The Ulster Unionist stance is also potentially problematic for the Conservative Party, which has an electoral pact with the UUP in Northern Ireland.

David Cameron is a supporter of the Hillsborough deal and he now faces the prospect of going to the polls aligned to a party which opposes it.

The UUP claim the Stormont Executive needs to demonstrate an ability to address other outstanding issues facing it — such as the uncertainty over education reforms — before it can be trusted with security powers.

After spelling out the party position on Friday, the UUP sent proposals to all Stormont's other parties in a bid to address its concerns.

A working group jointly chaired by Sir Reg to look at improving the working of the Executive meets this morning and the UUP leader said those discussions could have some impact on his party's thinking.